1. Field of Technology
The present invention relates to an object recognition apparatus installed in a vehicle, which transmits waves within a region ahead of the vehicle for detecting objects located in that region based upon resultant reflected waves, and in particular for detecting the widths of such objects.
2. Description of Related Art
Types of object recognition apparatus for installation in a vehicle (referred to in the following as the host vehicle) are known, whereby laser light beams or beams of millimeter-range radio waves are scanned within a region ahead of the host vehicle, with objects within that regions (in particular, other vehicles) being detected based on resultant received reflected waves. The detection results may be used to generate warning indications of an obstacle (such as a preceding vehicle) ahead of the host vehicle, or may be used in controlling the separation distance between the host vehicle and a preceding vehicle.
Such an object recognition apparatus may employ laser light beam scanning, in which pulse light beams are transmitted in successive azimuth directions (successive scan angles) to scan a predetermined angular region extending ahead of the host vehicle. By determining the range (distance from the host vehicle) of a detected object, the angular resolution of the scanning and the number of beams from which waves are reflected back from the object, the object width can be calculated.
Specifically, designating the angular resolution of scanning as θ(°), the width of a detected object as W, and the range of the detected object as Z, the following relationship can be established:W≅number of reflected beams)×θ×(π/180°)×Z 
However the above equation assumes that there is no overlapping between adjacent beams, whereas in actuality, it is not possible to transmit beams having ideal cut-off characteristics, such as illustrated in FIG. 10A. An actual transmitted beam is widened by having a wide low-intensity “skirt” region, in which intensity values are up to 0.5 times the peak intensity of the beam. This is illustrated in FIG. 10B, showing an example of the beam pattern of an actual transmitted beam. The effective angular beam width is thus greater than the ideal value.
As a result, overlapping occurs between adjacent beams, so that the above equation is not actually valid, since the angular resolution of scanning is reduced. Hence the width of a detected object may be estimated as greater than the actual width.
This effect of decreased resolution of beam scanning is more severe for laser light beam scanning than for scanning by millimeter-range electromagnetic waves.
A method intended to achieve more accurate estimation of detected object width, for such a beam scanning type of object recognition apparatus has been proposed in Japanese Patent Laid-Open No. 2002-022831. With that method, received-light signal strength values corresponding to reflected light from respective beams are each compared with a threshold value. Signal strength values below the threshold value are discarded (as resulting from overlapping of transmitted beams, scattering of light, etc.), and values above the threshold are judged to be valid. Each time a complete beam sweep is performed and resultant received signal data acquired for each of the beams, the data are examined to find if they express a detected object width that exceeds a predetermined maximum width. If the maximum width is exceeded, the threshold value is adjusted, in a direction for reducing the detected object width. In that way, as successive sweeps occur, the detected object width becomes limited to the predetermined maximum value. For example if the anticipated maximum width of objects to be detected is approximately 2.5 m, then the threshold is adjusted such that the maximum detected width is restricted to a somewhat larger value, e.g., 2.6 m.
This reference document also teaches that the peak value of received-light signal strength corresponding to a detected object may be obtained, and the aforementioned threshold set as a fixedly predetermined proportion of the peak value.
This reference document also teaches that the rate of change of a set of received-light signal strength data, at a point of intersection with the threshold value, can be evaluated and used to judge whether received signal data values exceeding the threshold value are to be used as valid data. Specifically, if the rate of change is more gradual than a specific degree, the data are discarded (are judged to result from scattering of light), while if the rate of change is sufficiently abrupt, signal strength values above the threshold value are accepted as being valid.
However that method has the following disadvantage. If the object to be recognized has low reflectivity, then the reflected light from the object may be judged to be scattered light, so that object recognition will not be achieved.
In addition, the following disadvantage occurs in the case of the above method whereby the threshold value is adjusted to restrict the maximum object width that is detected. A plurality of objects may be detected concurrently, with the objects being of respectively different widths and/or different ranges. In that case, as illustrated in FIGS. 5A, 5B (for the case of two detected objects), the received-light signal strength data corresponding to one of the objects may have a peak value substantially different from the peak value corresponding to the other detected object. In that case, there is only a single threshold value, adjusted appropriately with respect to the higher one of the peak values, then all of the received-light signal strength data corresponding to the other detected object may be below that adjusted threshold value. Hence, only the width of one of the two objects will be detected.
A similar disadvantage occurs in the case of the above method whereby the threshold is set as a fixed proportion of the peak value of received-light signal strength corresponding to a detected object. In that case too, since only a single threshold is utilized, it is not possible to achieve accurate width estimation of a plurality of objects which may be of various widths and/or located at various different distances, and so correspond to a plurality of respectively different peak values of received-light signal strength.